BH250-143b
Title
BH250-143b
Subject
Franconia Sandstone
Description
Major Minerals: quartz
Minor Minerals: glauconite, calcite, plagioclase feldspar, clay minerals
Texture: fine- to medium-grained, well- to moderately sorted, with rounded to sub-rounded grains.
Cement: primarily silica (quartz overgrowths), with minor iron oxide and clay cement in some layers.
The Franconia Sandstone is a prominent Upper Cambrian sedimentary unit exposed across southeastern Minnesota and adjacent parts of Wisconsin and Iowa. It is especially well displayed at classic outcrops such as Taylors Falls and Red Wing, where it forms striking bluffs and cliffs along the St. Croix River and Mississippi River valleys.
Stratigraphic Revisions
In recent years, Minnesota stratigraphic terminology has been updated. Much of what was formerly referred to as the Franconia Formation is now formally assigned to the Tunnel City Formation, bringing Minnesota nomenclature into closer agreement with that used in Wisconsin and Illinois. The two names cover overlapping stratigraphic intervals, but Tunnel City reflects newer lithostratigraphic mapping and subdivision of Cambrian strata.
Depositional Environment
The Franconia (Tunnel City) Sandstone was deposited in shallow-marine environments across a broad, low-gradient continental shelf. Slow sediment accumulation and marine circulation favored the development of glauconite, an authigenic mineral commonly associated with marine shelf settings.
At Taylors Falls (BH250-143), glauconite is present but less abundant than in the Red Wing area (BH250-143b), where glauconitic layers are more pronounced.
Sedimentary Features
Notable features, especially at Taylors Falls, include:
• planar and trough cross-bedding
• ripple marks
• bioturbation structures
• laminated bedding
These structures indicate active currents, shifting sand bars, and biologically disturbed seafloor sediments in a shallow-marine setting.
Diagenesis and Glauconite Rims
Unlike the glauconites in BH250-143, many glauconitic grains in BH250-143b display distinct rims. Glauconite grains commonly develop thin coatings when their original marine chemistry is modified during diagenesis. These rims likely formed through changing redox conditions and pore-fluid chemistry, involving oxidation or interaction with evolving interstitial waters. As a result, they record post-depositional processes such as reworking and chemical alteration.
These rims may be broadly compared with the quartz overgrowths observed in sandstone thin section BH250-178, where authigenic silica precipitated on detrital quartz grains during diagenesis. However, the processes differ significantly. Quartz overgrowths are crystallographic continuations of the original quartz grain formed under silica-saturated conditions, producing optically continuous rims. In contrast, glauconite rims are alteration or precipitation products formed under changing chemical conditions, and they commonly differ compositionally and optically from the grain core.
In contrast, BH250-69 from South Dakota also contains glauconite, but no rims have been observed, suggesting a different diagenetic pathway or more stable redox conditions that did not favor rim development.
For more information, see BH250 – Sandstone Overview with Petrographic Notes.
Minor Minerals: glauconite, calcite, plagioclase feldspar, clay minerals
Texture: fine- to medium-grained, well- to moderately sorted, with rounded to sub-rounded grains.
Cement: primarily silica (quartz overgrowths), with minor iron oxide and clay cement in some layers.
The Franconia Sandstone is a prominent Upper Cambrian sedimentary unit exposed across southeastern Minnesota and adjacent parts of Wisconsin and Iowa. It is especially well displayed at classic outcrops such as Taylors Falls and Red Wing, where it forms striking bluffs and cliffs along the St. Croix River and Mississippi River valleys.
Stratigraphic Revisions
In recent years, Minnesota stratigraphic terminology has been updated. Much of what was formerly referred to as the Franconia Formation is now formally assigned to the Tunnel City Formation, bringing Minnesota nomenclature into closer agreement with that used in Wisconsin and Illinois. The two names cover overlapping stratigraphic intervals, but Tunnel City reflects newer lithostratigraphic mapping and subdivision of Cambrian strata.
Depositional Environment
The Franconia (Tunnel City) Sandstone was deposited in shallow-marine environments across a broad, low-gradient continental shelf. Slow sediment accumulation and marine circulation favored the development of glauconite, an authigenic mineral commonly associated with marine shelf settings.
At Taylors Falls (BH250-143), glauconite is present but less abundant than in the Red Wing area (BH250-143b), where glauconitic layers are more pronounced.
Sedimentary Features
Notable features, especially at Taylors Falls, include:
• planar and trough cross-bedding
• ripple marks
• bioturbation structures
• laminated bedding
These structures indicate active currents, shifting sand bars, and biologically disturbed seafloor sediments in a shallow-marine setting.
Diagenesis and Glauconite Rims
Unlike the glauconites in BH250-143, many glauconitic grains in BH250-143b display distinct rims. Glauconite grains commonly develop thin coatings when their original marine chemistry is modified during diagenesis. These rims likely formed through changing redox conditions and pore-fluid chemistry, involving oxidation or interaction with evolving interstitial waters. As a result, they record post-depositional processes such as reworking and chemical alteration.
These rims may be broadly compared with the quartz overgrowths observed in sandstone thin section BH250-178, where authigenic silica precipitated on detrital quartz grains during diagenesis. However, the processes differ significantly. Quartz overgrowths are crystallographic continuations of the original quartz grain formed under silica-saturated conditions, producing optically continuous rims. In contrast, glauconite rims are alteration or precipitation products formed under changing chemical conditions, and they commonly differ compositionally and optically from the grain core.
In contrast, BH250-69 from South Dakota also contains glauconite, but no rims have been observed, suggesting a different diagenetic pathway or more stable redox conditions that did not favor rim development.
For more information, see BH250 – Sandstone Overview with Petrographic Notes.
Coverage
Location: Red Wing, Minnesota, USA
GPS Coordinates: 44°34'6.05"N, 92°31'46.56"W
GPS Coordinates: 44°34'6.05"N, 92°31'46.56"W
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 143b. Housed at Carleton College in Minnesota.
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-143b,” BH250 Mineralogy Teaching Collection, accessed April 26, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/170.